Fix "Issie ate my sheets"

Issie has had a long-standing UI problem where under some circumstances multiple repeated design sheet renames or changes, made before the first change has completed, can result in over-written sheets.

This release cures one version of this, which can be found where the sheet open button is pressed repeatedly before the sheet open operation has terminated. After the fix - if you click multiple times Issie will re-open the same sheet multiple times - taking even longer - but not destroying data.

It is hoped that this fix has cured the problem completely.

Until the UI is rewritten from scratch eliminating these problems note that patience is needed when you have large designs - Issie can take 10s to open a new sheet and you need to wait.

The UI is obviously not good in this respect - but a full rewrite must await enough time to complete it.

• fix problem where repeated pressing of open or rename sheet buttons can result in sheets overwritten.